High molecular weight polyoxymethylene (also known as polyacetal) is a well-known commercial product. It can be made, for example, according to the process described in U.S. Pat. No. 2,994,687, which involves passing dry, gaseous formaldehyde into an agitated hydrocarbon solvent containing an initiator (typically a quaternary ammonium salt) and small amounts of a molecular weight control agent, typically methanol. The polymer is isolated and its hydroxyl end groups, arising from chain transfer by reaction with methanol and adventitious water, are capped by reaction with an aliphatic anhydride such as acetic anhydride to stabilize the polymer against depolymerization and provide sufficient thermal stability to allow the polymer to be melt-processed. However, this end-capping step requires the use of a large excess of acetic anhydride, adding complexity and expense to the manufacturing process.
It would be desirable to use an aliphatic anhydride as the molecular weight control agent in the polymerization step, which would have the further advantage of yielding a partially end-capped polymer with fewer hydroxyl end groups. This partially end-capped polymer could then be fully end-capped in a post-polymerization process that is cheaper and more efficient than the processes currently used to end-cap all the hydroxyl end groups formed in the polymerization. However, when used with known quaternary ammonium salt initiators, such as dihydrogenated tallow dimethylammonium acetate (DHTA), the resulting polyoxymethylene polymer has a very small particle size. Small particles are disadvantageous as they are hard to handle, tend to clog filters and otherwise foul systems, have a lower bulk density, lead to lower process throughput and lower the capacity of a system.
U.S. Pat. No. 4,539,387, which is hereby incorporated by reference, discloses the use of a diquatemary ammonium salt as an initiator for the polymerization of formaldehyde to polyoxymethylene. The use of this initiator in the presence of an aliphatic anhydride in hydrocarbon solvents leads to polyoxymethylene with particle sizes that are significantly increased over those obtained when standard initiators such as DHTA were used. However, this increased particle size comes at the expense of a decreased rate of polymerization. It would be desirable to be able to use a polymerization initiator that both yielded polyoxymethylene with large particle sizes when used in the presence of aliphatic anhydride and that did not decrease the rate of polymerization relative to that obtained using the typical quaternary ammonium salt initiators known in the art.